A method is introduced to create complex architectures in biomolecular condensates through controlled nucleation. By inducing droplet nucleation in condensates due to limited diffusion and composition changes, researchers can design multiphase condensates with various mesoscale structures. These architectures are transient states formed out of equilibrium and can be used to enhance compartmentalization and functionality in condensates for biotechnology, artificial cells, and origin-of-life research. The study shows that the architecture of condensates depends on how temperature is changed, not the temperature itself. Rapid cooling leads to higher-energy transient architectures, while slower cooling allows for more stable structures. The diffusion-limited nucleation process is key to forming these complex architectures, and the study provides a detailed method for predicting and designing such structures. The results demonstrate that controlling the architecture of condensates can significantly influence the uptake rate of cargo molecules, as shown in drug-delivery applications. The study also highlights the importance of phase diagrams and tie lines in understanding and predicting condensate architectures. Overall, this approach offers a general strategy for creating complex structured condensates out of equilibrium, providing insights into the structure of condensates in cells.A method is introduced to create complex architectures in biomolecular condensates through controlled nucleation. By inducing droplet nucleation in condensates due to limited diffusion and composition changes, researchers can design multiphase condensates with various mesoscale structures. These architectures are transient states formed out of equilibrium and can be used to enhance compartmentalization and functionality in condensates for biotechnology, artificial cells, and origin-of-life research. The study shows that the architecture of condensates depends on how temperature is changed, not the temperature itself. Rapid cooling leads to higher-energy transient architectures, while slower cooling allows for more stable structures. The diffusion-limited nucleation process is key to forming these complex architectures, and the study provides a detailed method for predicting and designing such structures. The results demonstrate that controlling the architecture of condensates can significantly influence the uptake rate of cargo molecules, as shown in drug-delivery applications. The study also highlights the importance of phase diagrams and tie lines in understanding and predicting condensate architectures. Overall, this approach offers a general strategy for creating complex structured condensates out of equilibrium, providing insights into the structure of condensates in cells.